With the increasing down-scaling of integrated circuits and increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with smaller dimensions. Fin field-effect transistors (FinFET) were thus developed. FinFET transistors have increased channel widths, which channels include the channels formed on the sidewalls of the fins and the channels on the top surfaces of the fins. Since the drive currents of transistors are proportional to the channel widths, the drive currents of FinFETs are increased.
To maximize the channel width of a FinFET, the FinFET may include multiple fins, with the ends of the fins connected to a same source and a same drain. In conventional processes, the formation of a multi-fin FinFET include forming a plurality of fins parallel to each other, forming a gate stack on the plurality of fins, and interconnecting the ends of the plurality of fins to form a source region and a drain region. An epitaxy process may be performed to grow a semiconductor material so that the ends of the plurality of fins merge with each other to form block source and drain regions. Source and drain contact plugs are then formed to connect to the block source and drain regions. This method, however, suffers from gap filling problems. For example, the distance between the fins that belong to the same FinFET is typically very small. Accordingly, it is difficult to perform the gap-filling to fill a dielectric material into the space between the fins.